Semiconductor memory device and driving method of the same

ABSTRACT

A non-volatile semiconductor memory device that can reduce power consumption includes plural memory banks containing nonvolatile plural memory cells. A common data bus is shared by plural memory banks and transmits the data of the memory cells. The plural switches are provided respectively between the electric source and plural memory banks. A controller controls the plural switches. The controller, in the data reading-out action or the data writing-in action, makes at least one of the switches corresponding to at least one of the memory banks accessible in a conduction state, and other switches in a non-conduction state.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/665,680, filed on Mar. 23, 2015, which is a continuation of U.S.patent application Ser. No. 13/606,793, filed on Sep. 7, 2012, now U.S.Pat. No. 8,988,933, granted on Mar. 24, 2015, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2012-065370, filed Mar. 22, 2012, the entire contents of each of theforegoing are incorporated herein by reference.

FIELD

Embodiments described herein relate to a semiconductor memory device anda driving method of the same.

BACKGROUND

In general, DRAM (Dynamic Random Access Memory) and other volatilememories are currently designed to JEDEC (JEDEC Solid State TechnologyAssociation) specifications covering LPDDR2 (Low Power Double Data Rate2). In LPDDR2, the DPD (Deep Power Down) state is specified. In the DPDstate the DRAM element is cutoff from the outside electric source toreduce electric power consumption during the waiting state.

However, when the DRAM element transitions to the DPD state the datastored in the memory vanishes. Therefore, when the element returns tothe idle state (the electric passage state) from the DPD state, it isnecessary to rewrite (initialize) the data to the memory again.Therefore, it takes a relatively long time and a large electric powerconsumption to return from the DPD state. Thus, in a system using DRAM,or other volatile memories, the memory element is often maintained in anidle state, which has a shorter return time than the DPD state, insteadof the DPD state.

In the idle state, it is necessary to carry out a refreshing actionperiodically to maintain the data inside the memory. Therefore, even inthe idle state, electric power is still required to a certain extent.

During the transition from the idle state to the DPD state considerableelectric power is consumed when the electric power is supplied to thecutoff portion of the memory element. Therefore, if the DPD state isentered for just a short interval before the return to the idle state,significant electric power is consumed during the return and potentiallyno overall power savings will be realized from entering the DPD state.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an MRAM and a chip controller CCaccording to a first embodiment.

FIG. 2 is a block diagram showing the components of the MRAM of thefirst embodiment.

FIG. 3 is an illustrative diagram showing the components of a singlememory cell MC.

FIG. 4 is a block diagram showing the components of the MRAM of thefirst embodiment.

FIGS. 5A and 5B are tables showing the states of the switches SW0 to SW3of the MRAM of the first embodiment.

FIG. 6 is a flow diagram showing the operation of the MRAM of the firstembodiment.

FIG. 7 is a block diagram showing a modified example of the firstembodiment.

FIG. 8 is a block diagram showing the components of an MRAM according toa second embodiment.

DETAILED DESCRIPTION

In general, example embodiments related to the present disclosure willbe explained with reference to figures. The present embodiments areexamples and are not intended to limit the present disclosure. Otherembodiments may be in keeping with the present disclosure though notexplicitly described.

According to an embodiment, there is provided a semiconductor memorydevice which suppresses electric power consumption as well as improvingthe device's activation rate.

The semiconductor memory device according to the present embodiment isprovided with memory banks containing nonvolatile memory cells. A commondata bus is shared by the memory banks and the common data bus transmitsdata of the memory cells. Switches are arranged between the electricsource and memory banks. A controller controls the switches. Thecontroller, when reading data (a data reading-out action) or writingdata (a data writing-in action), places at least one of the switchescorresponding to at least one of the memory banks in a conduction stateand places other switches in a non-conduction state.

The following embodiments can be used appropriately in a variety oftypes of non-volatile memories like the magnetic random access memory(MRAM), the resistance random access memory (ReRAM), the phase-changerandom access memory (PRAM), the ferroelectoric random access memory(FeRAM), and so on. In the following embodiments, the MRAM will beexplained as an example of the resistance changing type of memory, butthe invention is not so limited.

First Embodiment

FIG. 1 is a block diagram showing the MRAM and a chip controller CCaccording to the first embodiment. The chip controller CC is providedwith the central processing unit (CPU), the read only memory (ROM), thestatic random access memory (SRAM), and the LPDDR2 controller. TheLPDDR2 controller outputs the chip selection signal CS, the clock enablesignal CKE, the command address signal CA, the clock signal CK, the dataDQ, the strobe signal DQS, the mask data DM and so on to the MRAM. Thechip controller CC controls the MRAM with these signals supplied by theLPDDR2 chip.

In general, the specifications of the LPDDR2 from JEDEC are used withDRAM and other volatile memories. However, in the present embodiment,the LPDDR2 specifications are applied an MRAM non-volatile memory.

The CPU controls the chip controller CC as a whole. The CPU includes themode register 10 and the SRAM 12. In regard to the mode register 10 andthe SRAM 12, a description will be given later.

FIG. 2 is a block diagram showing the components of the MRAM accordingto the first embodiment. The chip 1 of the MRAM is provided with thememory cell array MCA, the power generator PG, the logic circuit LC, theclock enable receiver RCKE, the command address receiver RCA, the databuffer DQB, and the input-output section I/O.

The memory cell array MCA is provided with, for example, plural memorycells MC in a 2-dimensional arrangement in a matrix form. The memorycell MC is a non-volatile memory cell and includes, for example, a MTJelement. Each memory cell MC is connected with a bit line pair (forexample, the bit line BL1 and the bit line BL2 shown in FIG. 2) and aword line WL. In other words, one end of the memory cell MC is connectedto one of the bit line pair (e.g., bit line BL1), and the other end isconnected to other bit line of the pair (e.g., bit line BL2). The bitline pair BL1 and BL2 extends in a direction perpendicular to the wordline WL.

The memory cell array MCA is further provided with the sense amp sectionSA, the write driver WD, the address decoders AD1 and AD2, the maincontroller MCNT, and the write-read buffer WRB.

The sense amp section SA is connected to, for example, the memory cellMC via the bit line BL1 and has the function of detecting the data inthe memory cell MC. The write driver WD is connected to, for example,the memory cell MC via the bit lines BL1 and BL2 and has the function ofwriting data to the memory cell MC.

The main controller MCNT transmits the data received from the DQ bufferDQB to the write driver WD so that the data are written to the desiredmemory cell MC. The main controller MCNT also transmits the data readfrom the desired memory cell MC to the DQ buffer DQB.

The address decoder AD1 is constituted according to the address from thecommand address receiver RCA or by the selection of the bit line pair.The address decoder AD2 is constituted according to the address from thecommand address receiver RCA or by the selection of the word line WL.

The write-read buffer WRB temporarily stores the write data (written-indata) input via the input-output section I/O and the data buffer DQB, ortemporarily stores the data read (read out data) from the memory cellMC.

The data buffer DQB temporarily holds written-in data and read out data,so the read out data can be output via the input-output section I/O orand the written-in data can be input to memory array via theinput-output section I/O.

The clock enable receiver RCKE receives the clock enable signal CKE forenabling the clock signal or not and, in the case of the activation ofthe clock enable signal CKE, the clock signal is transmitted.

The power generator PG generates an electric source voltage for thedriving of the memory cell array MCA. For example, the power generatorPG increases or decreases the electric source voltage supplied from theoutside to generate the electric source voltage VDD or the referencevoltage VSS.

The chip 1 is provided with the power controller PC, the commandcontroller COMCNT, the array controller ARYCNT, and so on. The powercontroller PC controls the power generator PG, the main controller MCNT,and the command address receiver RCA. The power controller PC can bringabout the selectively initiated state (the on state) or the paused state(the off state) of the power generator PG, the main controller MCNT, andthe command address receiver RCA.

The command controller COMCNT receives the commands for a variety ofactions such as the reading-out action from the command address receiverRCA, the reading-in action, and so on. The main controller MCNT iscontrolled according to these commands.

The command address receiver RCA receives commands and addresses forcontrolling actions of the memory cell array MCA. The command addressreceiver RCA receives, for example, the row address, the page address,and so on as addresses. The command address receiver RCA also receives,for example, the active command ACT, the power down command PD, the deeppower down command DPD, the MR write command MRW, the MR read commandMRR, the reset command RST, and so on as commands. With these commands,the memory cell array MCA can carry out a variety of actions.

The command controller COMCNT controls the array controller ARYCNT andso on according to the commands from the outside. The array controllerARYCNT, on the basis of data obtained from mode register MR, controlsthe MRAM inside of the memory banks BNK0 to BNK3, the switches SW0 toSW3, and so on.

FIG. 3 is an illustrative diagram showing the components of a singlememory cell MC. Each memory cell (MC) contains a magnetic tunneljunction (MTJ) element, and a cell transistor CT. The MTJ elementdepicted is a STT (Spin Transfer Torque)-MTJ element. The MTJ elementand the cell transistor CT are connected in series between the bit lineBL1 and the bit line BL2. In the memory cell MC, the cell transistor CTis installed on the BL2 side, and the MTJ element is installed on theBL1 side. The gate of the cell transistor CT is connected on the wordline WL.

The STT-MTJ element utilizing the TMR (tunneling magnetoresistive)effect has a laminated structure including two sheets of ferromagneticlayers with a nonmagnetic layer (the insulation thin layer) sandwichedbetween them. The STT-MTJ element records (memorizes) the digital databy a change in magnetic resistance according to the deflecting electrodetunnel effect.

The MTJ element acquires the low resistance state and the highresistance state by the magnetization arrangement of two sheets offerromagnetic layers. For example, if the low resistance state isdefined as data “0” and the high resistance state is defined as data“1,” 1 bit data can be recorded in the MTJ element. Of course, it isalso acceptable that the low resistance state is defined as data “1” andthe high resistance state is defined as data “0.”

The MTJ element, as shown in FIG. 3, may be, for example, constituted bylamination in the sequence of the fixation layer P, the tunnel barrierlayer B, and the recording layer Fr. The fixation layer P and therecording layer Fr are ferromagnetic material, and the tunnel barrierlayer B is an insulation film. The fixation layer P has a fixeddirection of magnetization. The recording layer Fr has a variabledirection of magnetization and the data are recorded by reference to thedirection of its magnetization.

If an electric current above the reverse threshold value current flowsin the direction of the arrow symbol A1 during writing, the recordinglayer Fr will be in an anti-parallel state with respect to the directionof the magnetization of the fixation layer P. In such a case, the MTJelement will be in the high resistance state (data “1,” according to thepresent scheme). If the electric current above the reverse thresholdvalue current flows in the direction of the arrow symbol A2 duringwriting, the recording layer Fr will be in a parallel state with respectto the direction of the magnetization of the fixation layer P. In such acase, the MTJ element will be in the low resistance state (data “0,”according to the present scheme). In this way, the MTJ element can writein different data according to the direction of the electric currentflow.

FIG. 4 is a block diagram showing schematically the components of theMRAM according to the first embodiment. FIG. 4 shows four memory banksBNK0 to BNK3 containing memory cells. The memory banks BNK0 to BNK3share the common data bus DB. The common data bus DB transmits the dataof the memory cells inside the memory banks BNK0 through BNK3 to thelogic circuit LC and the interface IF.

The memory banks BNK0 to BNK3 are connected to the electric source VDDvia the respective switches SW0 to SW3. By having the switches SW0 toSW3 on or off, the memory banks BNK0 to BNK3 receive the electric powerfrom the electric source VDD or are cut off from the electric sourceVDD. The switches SW0 to SW3 can be formed by using FETs (Field EffectTransistors).

The logic circuit LC controls the memory banks BNK0 to BNK3, theswitches SW0 to SW3, and so on. The interface IF includes the I/Ocircuit for outputting the data from the common data bus DB to theoutside of the memory chip 1 or for transmitting the data from theoutside of the memory chip 1 to the common data bus DB.

The mode register MR stores the action states of the MRAM and so on. Forexample, the mode register MR issues the inside command corresponding tothe DPD command, the inside command corresponding to the PAU (PartialArray Usage) command, the active command and determines the type ofburst, the burst length, the latency period and other MRAM actions. Themode register MR also stores the inside set values showing the MRAMstates.

The mode register MR also stores, beforehand, the memory size activatedby the PAU command. For example, in the mode register MR, activation ismade possible by the PAU command for each memory bank when setting thememory size of the PAU command in the memory bank MB.

The memory chip containing the memory banks BNK0 to BNK3, the interfaceIF, the logic circuit LC, the mode register MR, and so on is connectedto the electric source VDD via the main switch MSW. The main switch MSWcan cut off the electric power supply to the memory chip as a whole.

The mode register 10 inside the CPU of the chip controller CC in FIG. 1,stores the action states of the MRAM and so on in the same manner as themode register MR inside the memory chip 1. In accessing any of thememory banks BNK0 to BNK3, the MRW (Mode Register Write)-PAU command isissued. The MRW-PAU command is a command for writing the set value ofthe mode register MR in the PAU mode into the mode register MR. By theMRW-PAU command, the PAU command of the mode register 10 is written inthe mode register MR on the MRAM side. The MRAM, in accordance with thePAU command, accesses any of the memory banks BNK0 to BNK3. Furthermore,the SRAM 12 inside the CPU of the chip controller CC, depicted in FIG.1, stores the address information for the memory banks that are the onstate and are accessible and the memory banks that are in the off stateand are not accessible.

When the MRAM returns from the DPD state to the idle state or the activestate, the MRR (Mode Register Read)-PAU command is issued. In the DPDstate, electric power is not supplied to the mode register 10 or theSRAM 12. Therefore, the PAU command stored in the mode register 10 andthe SRAM 12 is lost. But the mode register MR on the MRAM side maintainsthe PAU command. Therefore, when the MRAM returns from the DPD state tothe idle state or the active state (by the MRR-PAU command) the data inthe mode register MR on the MRAM side can be read out to the moderegister 10 and the SRAM 12 on the CPU side.

The idle state is a state in which electric power is supplied to aselected memory bank (BNK0 in this example), and the commands for dataread out and data write in can be received by the selected memory bank.The idle state is different from the waiting state in which the electricpower to a bank is cut. In a deep power down, all memory banks will bein the waiting state. The memory bank BNK0 in the idle state is waitingfor commands (the command waiting state). If a command is received, thememory bank BNK0 will be in an active state.

FIG. 5A and FIG. 5B are tables showing the states of the switches SW0 toSW3 of the MRAM according to the first embodiment. FIG. 5A shows theon/off states of switches SW0 to SW3 in the deep power down state (theDPD state). FIG. 5B shows the on/off states of switches SW0 to SW3 whenthe MRAM is in the idle state (or the active state when accessing onlythe memory bank BNK0). In the idle state, the active state, and the DPDstate, the main switch MSW is in the on state, and the memory chip issupplied with power from the electric source.

As shown in FIG. 5A, when the MRAM of the present embodiment is in theDPD state, the switches SW0 to SW3 are in the off state (thenon-conduction state). Therefore, the memory banks BNK0 to BNK3 do notreceive electric power from the electric source VDD. Since the memorybanks BNK0 to BNK3 are nonvolatile, the data can still be maintainedeven without the refreshing action which would be required with DRAM. Inthis way, according to the present embodiment, the MRAM can decrease theelectric power consumption to the same extent as the DPD state of theLPDDR2 specifications (for the conventional DRAM).

Next, as shown in FIG. 5B, the memory bank BNK0 can be available for thedata reading-out or data writing-in action. In this case, the logiccircuit LC has the switch SW0 in the on state (the conduction state) andother switches SW1 to SW3 maintained in the off state (thenon-conduction state). In other words, the logic circuit LC has memorybank BNK0 selected in the data reading-out or data reading-in action inthe active state and the other unselected memory banks BNK1 to BNK3 inthe DPD state.

In order to actuate the switches SW0 to SW3 in this manner, in thepresent embodiment, the PASR (Partial Array Self Refresh) command of theLPDDR2 specifications is used as a partial activation signal. The PASRcommand was originally used for the partial self-refreshing action inDRAM; however, in the present embodiment, the PASR command is used as apartial activation signal for the activation of a subset of availableMRAM memory banks (BNK0 to BNK3 in this example). For example, the PASRcommand can be set so that one or more memory banks among the memorybanks BNK0 to BNK3 can be selected. Furthermore, for the PASR commandall of the memory banks BNK0 to BNK3 can also be selected by default ifno subset is specified.

Here, since the PASR command is used as the partial activation signal,the PASR command is also called the PAU command. The PAU command isissued from the outside controller of the memory chip. An inside commandis issued by the mode register MR on the basis of the PAU command fromthe outside to select the inside state or action of the MRAM.

For example, when accessing the memory bank BNK0, the MRAM receives thePAU command for the selection of the memory bank BNK0 and the DPD ejectcommand DPDX for exiting the DPD state. The logic circuit LC, on thebasis of the DPDX command and the PAU command, switches the switch SW0to the conduction state while maintaining switches SW1 to SW3 in thenon-conduction state. The switch SW0 corresponds to the selected memorybank BNK0. Since the selected memory bank BNK0 receives electric powerfrom the electric source VDD, the MRAM can read data from the selectedmemory bank BNK0 or write data to the selected memory bank BNK0. Thedata reading-out action and the data writing-in action are the same asthe publicly known actions.

As an example application, when MRAM according to a present embodimentis used in a portable machine instead of the DRAM, the MRAM maintainsthe DPD state (normal off) shown in FIG. 5A while waiting for user inputor requests rather than the idle state shown in FIG. 5B. In doing so,the electric power consumption by the MRAM during the waiting period canbe the same level as for the DPD state for DRAM. In general, the user ofthe portable machine often walks while holding the portable machine inthe waiting state. The period of the waiting state is very long.Therefore, the suppression of the electric power consumption in thewaiting state is tied with the suppression of the electric powerconsumption of the battery of the portable machine and a reduction inthe frequency of electric charging.

Furthermore, the MRAM, when performing the data reading-out action orthe data-writing-in action, activates the required memory bank BNK0 byusing the DPDX command and the PAU command and keeps the memory banksBNK1 to BNK3 not requiring accessing in the DPD state. In doing so,wasteful electric power consumption can also be reduced for the MRAMwhen returning to the idle state or the active state. Therefore, whenMRAM according to the present embodiment is used in a portable machine,the electric power drawn from the battery of the portable machine isfurther reduced and time between battery recharging is improved.

Of course, when all memory banks BNK0 to BNK3 inside the memory chipmust be accessed, it is acceptable that the logic circuit LC makes allthe switches SW0 to SW3 in the conduction state and all the memory banksBNK0 to BNK3 in the active state. In this case, the MRAM may make thePAU command at the default value.

In the present embodiment, the mode register MR receives electric powerfrom the electric source VDD as long as the main switch MSW is not inthe non-conduction state. Therefore, the mode register MR still receiveselectric power from the electric source VDD while the MRAM is in the DPDstate. Thus, even if it uses volatile-type storage, the mode register MRof the present embodiment can maintain its data. If the main switch MSWis switched to a non-conduction state, then upon its return to theconduction state, the data in the mode register MR must be reloaded fromoutside the MRAM since data previously stored in the mode register MR islost when the main switch MSW is turned off. At this time, theregistered value of the PAU command returns to the initial state (thedefault value).

FIG. 6 is a block diagram showing the actions of the MRAM according tothe first embodiment. FIG. 6 shows the actions of the MRAM when the DPDcommand is issued while the memory bank BNK0 is being accessed.

First of all, the MRAM is in the active state with the memory bank BNK0being accessed. In this case, the MRW-PAU command designating the memorybank BNK0 is issued (S10). In doing so, the set value of the moderegister MR in the PAU mode is written to the mode register MR. Here,the MRW-PAU command includes the address of the memory bank BNK0 andallows selective access to the memory bank BNK0. In doing so, the logiccircuit LC, with the memory banks BNK1 to BNK3 being maintained in theDPD state, shifts the memory bank BNK0 from the DPD state to the activestate. In more detail, for the logic circuit LC, with the switches SW1to SW3 being maintained in the off state, the switch SW0 is switchedover to the on state (S40, S42). In doing so, the memory banks BNK1 toBNK3 are cut off from the electric source VDD and power is supplied onlyto the memory bank BNK0.

The MRAM then accesses the selected memory bank BNK0 via the common databus DB and reads out the data or writes in the data (S50). At this time,the unselected memory banks BNK1 to BNK3 maintain the DPD state.

In the embodiment described above, the memory bank BNK0 is selectivelyaccessed in the data reading-out actions or the data writing-in actions.However, it is also acceptable that one or more memory banks (any or allof the memory banks BNK0 to BNK3) can be accessed. In this case, thememory bank(s) designated in the MRW-PAU command of the step S10 can bechanged.

In the case when the DPD command has not been issued (NO in S60), theMRAM continuously repeats steps S10 to S52. However, the MRW-PAU commandissued in step S10 may designate respectively different memory banksBNK0 to BNK3.

In the case when the DPD command is issued when memory bank BNK0 is inthe active state (YES of S60), the MRAM shifts from the active state tothe DPD state (S70). This DPD state is the same as that explained withreference to FIG. 5A. At this time, the data in the mode register 10 andthe SRAM 12 on the CPU side vanish.

Next, if the DPDX command is issued (S80), the MRR-PAU command is issued(S90). At this time, the mode register MR on the MRAM side maintains theset register values of the PAU command. Therefore, by the MRR-PAUcommand, the set values of the mode register MR on the MRAM side areread out to the mode register 10 and the SRAM 12 on the CPU side. Indoing so, access to the memory bank BNK0 is made possible (S100).

By having the switches SW0 to SW3 in the off state during the waitingperiod, the device puts the memory banks BNK0 to BNK3 virtually in theDPD state. By switching switch SW0 to the on state, memory bank BNK0 ismade accessible for data reading-out actions or data writing-in actions,by using the DPDX command and the PAU command in the on state. In doingso, the MRAM according to the present embodiment can reduce the electricpower consumption both during the waiting state and during the return tothe active state.

Modified Example 1

FIG. 7 is a block diagram showing a modified example of the firstembodiment. In this modified example, the tri-state buffers (to besimply called buffers hereafter) TSB0 to TSB3 are provided respectivelybetween the memory banks BNK0 to BNK3 and the common data bus DB. Thetri-state buffers TSB0 to TSB3 serve as separation elements and each canhave three states of theoretically high state, theoretically low state,and high impedance state (HiZ state).

The memory banks BNK0 to BNK3 share the common data bus DB. Therefore,when the memory bank BNK0 is being accessed indefinite signals may beoutput in the common data bus DB since the memory banks BNK1 to BNK3 arein the DPD state. The buffers TSB0 to TSB3 are provided as separationelements in order to suppress the indefinite signals from the unselectedmemory banks BNK1 to BNK3 while they are in the DPD state.

The buffers TSB0 to TSB3, in the data reading-out action and the datawriting-in action, are in the conduction state between the selectedmemory bank BNK0 for accessing and the common data bus DB and thenon-conduction state between the other memory banks BNK1 to BNK3 and thecommon data bus DB. By having the buffers set in this manner, the datapropagated on the common data bus DB do not render an adverse effect onthe unselected memory banks.

The logic circuit LC, as described above, is provided with the commandcontroller COMCNT, the array controller ARYCNT, and the multiplexer MUX.

The multiplexer MUX controls the state of the buffers TSB0 to TSB3according to the command controller COMCNT or the array controllerARYCNT. In the case of the selection of the selected memory bank BNK0,the multiplexer MUX has the buffer TSB0 in the conduction state (the lowimpedance state) and other buffers TSB1 to TSB3 in the non-conductionstate (the high impedance state).

In this manner, the indefinite signals from the unselected memory banksBNK1 to BNK3 to the common data bus DB are cut off, and the exchange ofthe data between the selected memory bank BNK0 and the common data busDB can be carried out.

Second Embodiment

FIG. 8 is a block diagram showing the components of the MRAM accordingto the second embodiment. The MRAM according to the second embodiment isfurther provided with the nonvolatile register MR-NV for backing up theinside data of the mode register MR. Other constitutions of the secondembodiment are the same as the corresponding constitutions of the firstembodiment.

The nonvolatile register MR-NV may also be constituted with the MRAM orother nonvolatile memories. The nonvolatile register MR-NV buffers thedata of the mode register MR. Therefore, even if the main switch MSW isin the non-conduction state, the inside data of the mode register MR aremaintained in the nonvolatile register MR-NV. Shortly after the returnof the main switch MSW to the conduction state, the nonvolatile registerMR-NV can reload the inside data to the mode register MR. Furthermore,by reloading the inside data of the nonvolatile register MR-NV to themode register MR, the mode register MR can again maintain the valuesalready set without initialization to the default values.

Thus, the MRAM according to the second embodiment, can also have themain switch MSW in the non-conduction state during the waiting period.When a memory bank needs to be accessed during the waiting period, theMRAM can load the inside data of the nonvolatile register MR-NV to themode register MR then access the selected memory bank. The access to theselected memory bank can be carried out in the same manner as the firstembodiment. Therefore, the second embodiment can achieve the same effectas the first embodiment.

Furthermore, the MRAM, according to the second embodiment can have themain switch MSW in the non-conduction state during the waiting period.Thus, the electric power consumption can be further reduced.

If it is in the same DPD state as in the first embodiment, the data ofthe mode register 10 and the SRAM 12 on the CPU side vanish. Therefore,as explained with reference to steps S90 and S100 in FIG. 6, after theissuing of the DPDX command, the MRR-PAU command is issued. By issuingthe MRR-PAU command, the set values of the mode register MR on the MRAMside are read out to the mode register 10 and to the SRAM 12 on the CPUside. In doing so, access to the memory bank BNK0 is made possible.

The MRAM according to the first and second embodiments in this mannercan be used instead of both existing nonvolatile ROM and volatileworking memory (DRAM).

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions.

What is claimed is:
 1. A system, comprising: a chip controller; and amagnetic random access memory (MRAM) including an interface circuitry, afirst memory cell array, and a second memory cell array, wherein thechip controller supplies a clock signal, a command/address signal, adata signal, and a data strobe signal to the MRAM, the interfacecircuitry of the MRAM receives a first command and a second command fromthe chip controller by the command/address signal, in response to thefirst command, the first memory cell array is coupled to a power sourceand the second memory cell array is decoupled from the power source, andin response to the second command, the first and second memory cellarrays are decoupled from the power source.
 2. The system according toclaim 1, wherein the interface circuitry causes the first memory cellarray to be coupled to the power source and the second memory cell arrayto be decoupled from the power source in response to the first command,and the interface circuitry causes the first and second memory cellarrays to be decoupled from the power source in response to the secondcommand.
 3. The system according to claim 1, wherein the chip controllerincludes a first mode register configured to store operation states ofthe MRAM.
 4. The system according to claim 3, wherein the MRAM includesa second mode register configured to store operation states of the MRAM.5. The system according to claim 4, further comprising a non-volatileregister capable of storing data from the second mode register andsupplying stored data to the second mode register.
 6. The systemaccording to claim 4, wherein the first mode register is decoupled fromthe power source during a deep power down state and data stored in thesecond mode register is supplied to the first mode register after acommand to end the deep power down state is received.
 7. The systemaccording to claim 6, wherein the second mode register is coupled to thepower source during the deep power down state.
 8. The system accordingto claim 6, wherein the second mode register is decoupled from the powersource during the deep power down state and data from the second moderegister is stored in a non-volatile register during the deep power downstate.
 9. The system according to claim 1, wherein the MRAM includes amode register configured to store operation states of the MRAM.
 10. Thesystem according to claim 9, further comprising a non-volatile registercapable of storing data from the mode register and supplying stored datato the mode register.
 11. A semiconductor memory device, comprising: afirst memory cell array including a plurality of magnetic tunneljunction (MTJ) elements; a second memory cell array including aplurality of MTJ elements; an interface circuitry that is coupled to aclock signal, a command/address signal, a data signal, and a data strobesignal; and a control circuitry that is coupled to a power source, theinterface circuitry, and the first and second memory cell arrays,wherein in a first operation state, the control circuitry causes thefirst memory cell array to be connected to the power source and to causethe second memory cell array to be disconnected from the power source,and in a second operation state, the control circuitry causes both ofthe first and second memory cell arrays to be disconnected from thepower source.
 12. The semiconductor memory device according to claim 11,wherein the first operation state is an active state or an idle state.13. The semiconductor memory device according to claim 12, wherein thesecond operation state is a deep power down state.
 14. The semiconductormemory device according to claim 11, wherein the control circuitrycomprises first and second switches, the first switch being coupledbetween the first memory cell array and the power source, and the secondswitch being coupled between the second memory cell array and the powersource.
 15. The semiconductor memory device according to claim 11,further comprising a first mode register configured to store operationstates of the semiconductor memory device.
 16. The semiconductor memorydevice according to claim 15, further comprising a second mode registerconfigured to store operation states of the semiconductor memory device.17. The semiconductor memory device according to claim 16, furthercomprising a non-volatile register capable of storing data from thesecond mode register and supplying stored data to the second moderegister.